This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Indium oxide (In2O3) forms the basis of most of the transparent conducting oxides (TCO's) in use today. For example, indium-tin oxide (ITO) has found wide application in flat panel displays, solar glass, and energy efficient window coatings. Significantly, ITO exhibits a combination of excellent optical and transport properties as well as chemical stability.
ITO films can be deposited by various techniques, including sputtering, chemical vapor deposition, and by atomic layer deposition (ALD). In various applications, it can be helpful for device performance to have precise control over film thickness and composition, and some applications require the ability to coat high aspect ratio geometries or porous materials. The ALD process allows surfaces without line-of-sight access to the precursor sources to be coated with great uniformity. ALD also affords excellent control over both the thickness and the composition of the deposited film. These advantages facilitate the synthesis of TCO films for various applications, for example, nanostructured photovoltaics.
Although various techniques of ITO deposition by ALD have been developed, ALD of ITO films has generally not yet found commercial application. Indium oxide (In2O3) may be deposited using InCl3 with either H2O or H2O2 as the oxygen source. Although useful for coating planar surfaces, this method suffers from several limitations. First, the InCl3 chemistry requires high growth temperatures of ˜300-500° C. and yields a low growth rate of only 0.25-0.40 Å/cycle. In addition, the InCl3 has a low vapor pressure and must be heated to 285° C. just to saturate a planar surface. Furthermore, the corrosive HCl byproduct can damage deposition equipment. But the greatest limitation of the InCl3/H2O method, especially for coating nanoporous materials, is that InCl3 can etch the deposited In2O3. Consequently, the very long InCl3 precursor exposures necessary to coat nanoporous materials can completely remove the In2O3 from the outer portions of the nanoporous substrate.
Alternative ALD processes for In2O3 and ITO have been sought for many years and a number of alternate precursors have been investigated including β-diketonates (In(hfac)3 (hfac=hexafluoropentadionate), In(thd)3 (thd=2,2,6,6-tetramethyl-3,5-heptanedioneate), and In(acac)3 (acac=2,4-pentanedionate)) and trimethyl indium, (In(CH3)3). Unfortunately, these efforts were unsuccessful. No growth was observed using β-diketonates with water or hydrogen peroxide, while trimethyl indium did not yield self-limiting growth.
An improved ALD process for In2O3 and ITO is described in U.S. Pat. No. 7,709,056. The process utilizes alternating exposures to cyclopentadienyl indium (InCp) and ozone (O3) to deposit monolayers of indium oxide, and alternating exposures to tetrakisdimethylamino tin and hydrogen peroxide to deposit monolayers of tin oxide. A potential limitation of a process that utilizes ozone is that the ozone precursor can decompose on hot surfaces and this may reduce ozone concentration in the ALD system, leading to thickness and composition non-uniformities. For example, in scaled applications where larger or multiple substrates are processed, thickness variation of ITO film deposited on the substrate can vary up to 33% over a length of 18 inches along the flow axis of the ALD reactor and up to 50% over a 11 inch span along the longitudinal axis. However, while an InCp/O3 regime can achieve an acceptable growth rate of about 1.3 Angstrom per cycle, other oxidizing precursors including O2, H2O, H2O2, and N2O, exhibit poor growth rates of about 0.16, 0.068, 0.039, and 0.065 Angstrom per cycle at deposition temperatures of between about 250-300° C. and exhibit no or virtually no growth at lower deposition temperatures.
Additionally, various new applications require the ability to deposit ITO, for example, at lower temperatures and/or onto higher aspect ratio substrates. Still further, these and existing applications would benefit from processes with improved economics for various commercial and large volume applications. Accordingly, there is a need to improve existing processes for deposition of TCO materials.